The invention relates to a non-cryogenic separation process for separating components in a gaseous mixture. More particularly, it relates to improved pressure swing adsorption (PSA), vacuum swing adsorption (VSA) or temperature swing adsorption (TSA) process utilizing composite adsorbent particles.
Cryogenic air separation plants utilize ultra-low temperatures to separate oxygen and nitrogen from air. Such plants typically utilize low amounts of power, but have high capital costs associated therewith. Because of such high capital costs, cryogenic air separation plants are competitive only for high flow capacity and higher purity operations. For lower capacity operations, where cryogenic air separation plants may not be economically feasible, non-cryogenic processes are particularly suitable for a wide variety of important commercial applications. For example, air which has been substantially enriched in oxygen is used in various industries, such as chemical processing, steel mills, paper mills, and in lead and gas production operations. Nitrogen is also used in numerous chemical processing, refinery, metal production and other industrial application.
In the typical non-cryogenic adsorption process, a feed gas mixture, such as air, containing a more readily adsorbable component and a less readily adsorbable component, e.g. the nitrogen and oxygen components of air, is passed to the feed end of an adsorbent bed capable of selectively adsorbing the more readily adsorbable component at a higher adsorption pressure. The less readily adsorbable component passes through the bed and is recovered from the discharge end of the bed. Thereafter, the bed is depressurized (PSA), heated to higher temperature (TSA) or evacuated (VSA) to enable desorption of the more readily adsorbable component from the adsorbent and its removal from the feed end of the bed prior to the introduction of additional quantities of the feed gas mixture for treatment in a cyclic adsorption-desorption operation. Such processing is commonly carried out in multi-bed systems, with each bed employing the PSA, VSA or TSA processing sequence on a cyclic basis interrelated to carrying out of such processing sequence in the other beds of the adsorption system.
In PSA systems for the recovery of higher purity oxygen product as the less readily adsorbable component of air, each adsorbent bed will commonly contain an adsorbent material capable of selectively adsorbing nitrogen as the more readily adsorbable component, with the selectively adsorbed nitrogen being subsequently desorbed and recovered from the bed upon reduction of the pressure of the bed from the higher adsorption pressure level to a lower desorption pressure level. PSA systems for the recovery of nitrogen product have likewise been based on the use of adsorbents that selectively adsorb oxygen from air as the more readily adsorbable component thereof.
Early air separation systems, in particular, PSA systems, utilized two or more beds, with well-known molecular sieves, e.g. 13X zeolite molecular sieve material, being used as the adsorbent therein. Such zeolitic molecular sieve material, and other such materials, e.g. 5A material, capable of selectively adsorbing nitrogen from air, are equilibrium type adsorbents. Thus, an adsorption front of the selectively adsorbed nitrogen is formed at the feed end of the bed of such material, and advances toward the discharge or oxygen product end as a result of the equilibrium conditions established in the bed of zeolite molecular sieve material between the more readily adsorbable nitrogen and the less readily adsorbable oxygen components of feed air.
While conventional zeolite molecular sieves can be used effectively in PSA operations, specially modified materials can also be used for the desired selective adsorption of nitrogen from feed air, and the recovery of oxygen or nitrogen as the desired product gas. Thus, lithium cation exchanged forms of zeolite X have been developed for use in PSA and VSA systems. Such lithium exchanged zeolites, e.g. LiX adsorbent are found to exhibit highly desirable capacities and selectivities toward the adsorption of nitrogen from air or other streams containing less polar or less polarizable molecular species such as oxygen.
U.S. Pat. No. 4,859,217 discloses that very good adsorptive separation of nitrogen from oxygen can be obtained at temperatures of 15.degree. C. to 70.degree. C. using a type X zeolite which has more than 88% of its ions present as lithium ions, particularly when a zeolite having a silicon to aluminum atomic ratio of 1 to 1.25 is used. Lithium, divalent ion forms of zeolite type X (for example those described in U.S. Pat. No. 5,179,979) and lithium, trivalent ion exchanged forms of zeolite type X (as described in U.S. Pat. No. 5,464,467) also have excellent capacities and selectivities for nitrogen adsorption from oxygen, and have other advantages over Li-X, including better thermal stabilities.
While economical non-cryogenic gas separation processes have been developed, a constant challenge remains to improve the efficiency of these processes. Most research directed to meeting this challenge has been focused on improving the capacity and/or selectivity of the adsorbents. The present invention is directed to a unique approach to meeting this challenge without necessarily improving the capacity and selectivity of the adsorbent being used.